Conventional passive noise control approaches, such as sound absorbers or blockers, are typically either gigantic or heavy, especially for the low frequency noise control. Conventional active noise control provides another noise control option, but, its wiring and power requirement can make conventional active noise control costly, complex and hard to implement.
Further, conventional composite acoustic attenuation concepts are too heavy and bulky for certain applications. Some approaches rely on structural tension and lack stiffness control, or can be heavy if many masses are involved. Yet others have an operating frequency that is high, which makes it less effective for low-frequency operation. Another problem encountered with some conventional structures is that preciseness can be difficult to achieve as the environmental temperature changes.
The conventional noise control materials such as foams, blankets, barriers, and Helmholtz resonators rely either on homogenized material properties or dynamic behavior to reduce the noise. In the homogenized property category, the bulk materials reflect acoustic energy based on the mass law which depicts 6 dB noise reduction as doubling the frequency or surface density and the absorbent materials dissipate the energy with comparable thickness with at least one quarter of wave length. For low frequency noise control applications, the materials or the structure must be either extreme bulky and heavy to be able to provide adequate noise reduction and hence impractical for lightweight and compact requirements of modern vehicle design. As for the dynamic approaches, structural or acoustic resonators are constructed to control the noise with designated stop band frequency range; however, it is usually narrow band and less effective as frequency decreases. Further, since structural resonators with low stiffness such as membrane or thin plate often rely on structural tension to increase operation frequency which is often sensitive to environment temperature, tensioned structures resonators suffer from frequency drifting for applications with serious temperature change.
Thus, what is needed is a lightweight and compact design that is broadband and effective at low frequencies. Further, what is needed is a technology that reduces manufacturing costs and reduces environmental sensitivity to temperature or moisture.